1. Field of the Invention
The present invention relates to an image projection apparatus, such as a liquid crystal projector, etc., and particularly relates to an image projection apparatus with an autofocusing function.
2. Description of the Related Art
With regard to an AF (Auto Focus, referred to hereinafter as “AF”) operation of a conventional projector, an AF operation is started when a focus switch is pressed.
Known AF techniques include active methods, wherein AF operation is carried out upon performing distance determination by measuring the propagation time of an ultrasonic signal or based on the principles of triangulation using infrared light, and passive methods, wherein a lens is driven upon reading the contrast of brightness across a screen by means of a pair of light receiving line sensors and determining the distance by obtaining the correlation values of the respective pixel outputs, etc., (see Patent Document 1).
The abovementioned passive methods can be largely classified into two types, one type being the so-called two-image correlation (or displacement detection) AF method, wherein a pattern which is prepared in advance is read and projected and used as an AF chart to improve the precision of AF (see Patent Document 2), and the other type being the AF method called sharpness detection or contrast detection (hill-climbing) method, wherein the AF operation is performed by using an optical sensor with a one-dimensional or two-dimensional pixel configuration, and this type is used in many digital cameras and home videos.
Here, in the case where a projection light of the projector is used for focusing, though there will be no problem if the light source can provide adequate brightness without waiting from the turning ON of power, in the case where a high-pressure discharge lamp or other light source with a long required lighting time is used, there was the possibility for an AF operation being performed even when the brightness is inadequate for passive AF, thereby leading to erroneous AF operation.
The example proposed in Patent Document 3 may be cited as an example of an AF operation of an image reading apparatus wherein the AF operation is prohibited upon judging that the circumstances present a problem in terms of the precision of the AF operation.
Specifically, in an image reading apparatus having a reading unit that scans and reads in an enlarged projected image of a microfilm, the region in which the image is to be read is set and the reading unit performs sampling for autofocusing within the set region. A projection lens for projecting the enlarged projected image is made removable and when the projection lens is removed, the AF operation is restricted. Further, the AF operation is restricted when the occurrence of a system error is detected and the AF operation is restricted when the projection lens is not attached.                [Patent Document 1] Japanese Patent Application Laid-Open No. H4 (1992)-338707        [Patent Document 2] Japanese Patent No. 3120526        [Patent Document 3] Japanese Patent Application Laid-Open No. 2000-295443        [Patent Document 4] Japanese Patent Application Laid-Open No. H11 (1999)-109214        
However, arts concerning reductions of the time required for ranging and driving operation have hardly been proposed for AF mechanisms of the projection type image display apparatus such as the projector. This is because with an image projection apparatus, AF operation is often performed just once at the beginning and arts for speeding up an AF process were thus not viewed with much importance.
Propositions have also not been made in regard to speeding up optimal AF and specifically in regard to a driving method for setting the initial lens position in a projector, which has unique equipment application restrictions in terms of usage conditions, projection size conditions, etc., and there were inadequacies in terms of repressing unnecessary operation and erroneous operation by judgment of the circumstances during an AF operation.
The abovementioned Patent Document 4 discloses a projector wherein, for the purpose of initialization for subsequent use, a lens is driven to a predetermined position when the power is turned off. However, with the projector, the abovementioned function does not operate when the power is turned off hastily during the time for waiting for the end of cooling, etc. Furthermore, in the arrangement of the projector of the abovementioned Patent Document 4, initialization is performed when full completion is carried out from the standby state that is entered after the end of cooling.
The above arrangement thus has the problem that initialization is not carried out when the power is turned off hastily during the time for waiting for the end of cooling, etc. Also, in the case where full completion is carried out normally from the standby state after the end of cooling, the full completion time may become elongated due to standby drive. Difficulties thus exist in either case.